Oh, this is clever: the latest development build of TextMate includes a tool that lets you present dialog boxes using nib files for the UI (i.e. you can make them using Interface Builder). It’s not a C/Obj-C API, though — it’s just a simple shell tool that gives you the state of every control in the dialog as a property list when you close the window, so you can drive the whole thing from a simple Perl/Python/Ruby/shell script.

I’m not aware of any other “present a dialog box using a nib file” scripting implementation like this.